Vector network analyzers (VNAs) measure performance characteristics of a device under test (DUT) in terms of S-parameters. In particular, the VNA applies a radio frequency (RF) stimulus signal to one or more ports of the DUT and measures one or more of a reflected response and a transmitted response at each of the DUT ports. The VNA then employs a comparison of the applied RF stimulus signal to the measured responses to calculate the S-parameters for the DUT. The S-parameters, typically determined as a function of frequency, characterize a performance of the DUT.
Traditionally, VNAs often employed a continuous wave (CW) signal as the applied RF signal. As such, the VNA essentially measured a CW response of the DUT and produced S-parameters from the measured CW response. While CW stimulus signals and/or CW measurements may be acceptable in many cases, for some situations a pulse modulated (i.e., ‘pulsed’) applied RF stimulus signal and/or a pulsed measurement of the DUT response may be preferred or even required. For example, pulsed signals and/or pulsed measurements may be preferred when testing high-power microwave amplifiers to prevent damage to the DUT due to overheating. Similarly, when evaluating a performance of an active array antenna designed to operate with pulsed signals, pulsed RF stimulus signals may be the only practical alternative. Therefore, modern VNAs often provide for measuring S-parameters of such ‘pulsed’ DUT by using pulsed RF stimulus signals and/or by employing pulse-oriented response measurements (pulsed measurements) of the DUT.
In general, approaches to measuring so-called ‘pulsed DUTs’ with a VNA can be broken down into two main categories: broadband and narrowband. VNAs in the broadband category utilize broadband receivers having rapid rise times and either fast synchronous detectors or high-speed analog-to-digital converters (ADCs). The so-called broadband VNAs utilize the broadband receivers to sample, capture and process with minimal distortion narrow pulses received from the DUT and/or an RF stimulus source. VNAs in the narrowband category typically employ narrowband receivers synchronized to a pulse repetition frequency (PRF) of the pulsed DUT. In particular, the narrowband receivers are synchronized to the PRF by filtering the received pulsed signal to preserve a fundamental frequency and to remove harmonic frequencies associated with the pulsed signal.
Unfortunately, broadband VNAs are typically limited in terms of a minimum pulse width by an ultimate bandwidth performance (e.g., receiver rise-time, detector/ADC speed, etc.) that is achievable given the performance, availability and cost of broadband receiver components. On the other hand, while typically having less stringent requirements for receiver component performance than broadband VNAs, narrowband VNAs are limited to a minimum PRF by a stopband performance limit of a bandpass filter used in processing the received signal pulses.
Accordingly, it would be advantageous have a VNA for measuring pulsed DUTs that employed narrowband receivers and did not suffer from the stopband performance limit of the bandpass filter. Such a pulsed measurement VNA would solve a long-standing need in the area of pulsed RF measurement of DUTs.